Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MRI systems require a uniform main magnetic field, Bo, in the imaging volume that should remain homogeneous and constant in time over a wide range of pulse sequences and protocols. Changes or drift in the main magnetic field can affect the performance of the MRI system including data acquisition and reconstruction of an MR image. During a patient scan, the gradient coil(s) of the gradient coil assembly, which produce the magnetic field gradients, dissipate large amounts of heat. The heat produced by the gradient coils can cause an increase in temperature of the magnet warm bore, for example, by radiation, convection or conduction heating. In addition, the magnet warm bore temperature may increase as a result of eddy currents. A magnet warm bore surface is typically made of low magnetic permeability stainless steel, however, the stainless steel may have residual permeability, also known as paramagnetism. According to Curie's law, the permeability of a paramagnetic material changes as the temperature of the material changes. Accordingly, the heating of the magnet's stainless steel warm bore due to the heat generated by the gradient coils and eddy currents changes the permeability of the stainless steel warm bore. Typically, the permeability of the warm bore will decrease as the temperature of the warm bore increases. The change in permeability of the magnet warm bore can result in a change or drift in the main magnetic field which in turn can have a negative impact on image quality.
It would be desirable to provide a system, method and apparatus for controlling the change or drift of the main magnetic field. It would be advantageous to control or compensate for the change or drift of the main magnetic field based on the temperature and permeability of the magnet warm bore.